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In CCTV footage obtained during the incident, Mr Rowley can be seen paying cheques into an account when Fernandez walks in wearing a motorcycle helmet and brandishing a fake gun, before demanding the cashier serving Mr Rowley hand over cash.

Mr Rowley told The Evening Standard he realised the gun was a fake when he spoke to Fernandez, who he said appeared to hesitate.

He then shouted “a few choice words” at Fernandez before running out of the branch and down the street after him.

“I was in a bad mood," Mr Rowley explained. "I’d been dashing around all morning like a headless chicken trying to get the cheques and money together to pay the bills.

“I was at the counter and I saw this guy walk in with a crash helmet on. I just thought ‘here we go’. I knew what was going to happen next and I wasn’t in the mood for it.

“I’ve never been in a situation like that before, I just acted on instinct. I chased him up the road with a fire extinguisher and when I caught up with him he hit me over the head with his crash helmet and knocked me to the floor.

“My friend grabbed him and we both held onto him until the police turned up. By that point he knew he wasn’t going anywhere.”

He told officers he had seen Fernandez throw an object into a nearby building site, which was the imitation fire arm. Police discovered Fernandez, who was homeless, had also committed two identical raids in Barnet and Watford earlier in 2013.

He pleaded guilty two counts of robbery and one attempted robbery at Kingston Crown Court on Monday and was sentenced to eight years in prison.

Mr Rowley has been presented with a Commendation for his actions by Assistant Commissioner Mark Rowley and will be awarded a Police Public Bravery Award at the Association of Chief Police Officers annual awards ceremony on Thursday.

Detective Sergeant David Barnfather, from the Met’s Flying Squad, said: “The bravery of James Rowley was truly remarkable in confronting what was for all intents and purposes, an armed and extremely dangerous criminal.

“His courage and sense of civic duty are to be applauded and thanks to him Fernandez is behind bars".